The present invention relates to power supplies. It finds particular application in conjunction with power supplies for x-ray tubes and will be described with particular reference thereto.
The current through the filament of an x-ray tube is one of the parameters that is controlled during an x-ray exposure. Most commonly, the filament is connected in series with a constant current supply. A current limiting or controlling device is placed in series between the constant current source and filament for controlling the amount of current flowing through the filament. The amount of current controls the amount of energy put into the filament, hence its temperature. The temperature affects the rate at which electrons are boiled off, hence the tube current or electron flow between the cathode and anode. If the x-ray tube were operated while the filament is overheated, the patient would be over irradiated and the anode could be damaged.
Commonly, the filament is only brought up to temperature for an exposure and is at a reduced temperature between exposures. One of the problems with a constant current source is that it brings the filament up to the selected operating temperature relatively gradually.
One solution for bringing the filament up to its operating temperature is described in U.S. Pat. No. 4,775,992 to Resnick and Dupuis. To bring the filament up to temperature more quickly, a current boost is applied when the current to the filament is first turned on. That is, instead of supplying the normal operating current to the filament, a higher current is provided for a preselected short duration. Commonly, a current limiting device is disposed between the current source and the filament to prevent the filament from being overdriven. That is, the current limiting device keeps the filament from receiving a current that would heat the filament to a temperature at which a tube current is produced that will heat damage the anode. If such a current boost pulse were applied after the filament were already up to temperature, the filament would become overheated and the resultant tube current could thermally damage the anode. The current limiting device is provided in the circuit to prevent such thermal damage.
To prevent the current limiting device from blocking the boost current, the current limiting device is deactivated during the current boost pulse. One of the drawbacks of this technique is that the current boost or overshoot must be calculated accurately. If the current boost pulse is too high or too long, the filament overheats. If the filament is still hot from the preceding exposure, the filament will overheat. If the pulse is too short or too low, only a minimal increase in the filament heating rate is achieved. Moreover, if the current limiting circuit fails to be reactivated after the current boost is over, the filament is not protected against an overcurrent.
In some x-ray tube power supplies, a voltage source rather than a current source drives the filament. With a voltage source, the power delivered to the filament is proportional to the V.sup.2 /R, where V is the voltage and R is the filament resistance. Because the filament resistance is low, when the filament is cool and increases as the filament becomes warmer, the actual current flowing through the filament from a constant voltage source is higher initially and drops off towards the steady state operating current as the filament warms. This provides a built-in protection against overheating a filament still hot from the preceding exposure. Although a voltage source brings the filament up to temperature more quickly. It is relatively difficult to control. Typically, the x-ray tube current is relatively high and its resistance relatively low. Moreover, the same current flows through relatively long power supply cables between the power supply and the x-ray tube. The resistance of the cables tends to exceed the resistance of the filament. Because the actual current supply is controlled by the V.sup.2 /R relationship in which R is the sum of these resistances, the filament current control accuracy is much worse than with a constant current source.
In accordance with the present invention, there is provided a power supply which has both the filament warm-up advantages of a constant voltage source and the filament current control advantages of a constant current source.